1. Field of the Invention
This invention relates generally to a variable optical attenuator (VOA), and more particularly to an all-fiber acousto-optic tunable intensity attenuator that is useful in optical telecommunications systems.
2. Description of Related Art
In modern telecommunication systems, many operations with digital signals are performed on an optical layer. For example, digital signals are optically amplified, multiplexed and demultiplexed. In long fiber transmission lines, the amplification function is performed by Erbium Doped Fiber Amplifiers (EDFA""s). The amplifier is able to compensate for power loss related to signal absorption, but it is unable to correct the signal distortion caused by linear dispersion, 4-wave mixing, polarization distortion and other propagation effects, and to get rid of noise accumulation along the transmission line. For these reasons, after the cascade of multiple amplifiers the optical signal has to be regenerated every few hundred kilometers. In practice, the regeneration is performed with electronic repeaters using optical-to-electronic conversion. However to decrease system cost and improve its reliability it is desirable to develop a system and a method of regeneration, or signal refreshing, without optical to electronic conversion. An optical repeater that amplifies and reshapes an input pulse without converting the pulse into the electrical domain is disclosed, for example, in the U.S. Pat. No. 4,971,417, Radiation-Hardened Optical Repeaterxe2x80x9d. The repeater comprises an optical gain device and an optical thresholding material producing the output signal when the intensity of the signal exceeds a threshold. The optical thresholding material such as polydiacetylene thereby performs a pulse shaping function. The nonlinear parameters of polydiacetylene are still under investigation, and its ability to function in an optically thresholding device has to be confirmed.
Another function vital to the telecommunication systems currently performed electronically is signal switching. The switching function is next to be performed on the optical level, especially in the Wavelength Division Multiplexing (WDM) systems. There are two types of optical switches currently under consideration. First, there are wavelength insensitive fiber-to-fiber switches. These switches (mechanical, thermo and electro-optical etc.) are dedicated to redirect the traffic from one optical fiber to another, and will be primarily used for network restoration and reconfiguration. For these purposes, the switching time of about 1 msec (typical for most of these switches) is adequate; however the existing switches do not satisfy the requirements for low cost, reliability and low insertion loss. Second, there are wavelength sensitive switches for WDM systems. In dense WDM systems having a small channel separation, the optical switching is seen as a wavelength sensitive procedure. A small fraction of the traffic carried by specific wavelength should be dropped and added at the intermediate communication node, with the rest of the traffic redirected to different fibers without optical to electronic conversion. This functionality promises significant cost saving in the future networks. Existing wavelength sensitive optical switches are usually bulky, power-consuming and introduce significant loss related to fiber-to-chip mode conversion. Mechanical switches interrupt the traffic stream during the switching time. Acousto-optic tunable filters, made in bulk optic or integrated optic forms, (AOTFs) where the WDM channels are split off by coherent interaction of the acoustic and optical fields though fast, less than about 1 microsecond, are polarization and temperature dependent. Furthermore, the best AOTF consumes several watts of RF power, has spectral resolution about 3 nm between the adjacent channels (which is not adequate for current WDM requirements), and introduces over 5 dB loss because of fiber-to-chip mode conversions.
Another wavelength-sensitive optical switch may be implemented with a tunable Fabry Perot filter (TFPF). When the filter is aligned to a specific wavelength, it is transparent to the incoming optical power. Though the filter mirrors are almost 100% reflective no power is reflected back from the filter. With the wavelength changed or the filter detuned (for example, by tilting the back mirror), the filter becomes almost totally reflective. With the optical circulator in front of the filter, the reflected power may be redirected from the incident port. The most advanced TFPF with mirrors built into the fiber and PZT alignment actuators have only 0.8 dB loss. The disadvantage of these filters is a need for active feedback and a reference element for frequency stability.
A VOA is an opto-mechanical device capable of producing a desired reduction in the strength of a signal transmitted through an optical fiber. Ideally, the VOA should produce a continuously variable signal attenuation while introducing a normal or suitable insertion loss and exhibiting a desired optical return loss. If the VOA causes excessive reflectance back toward the transmitter, its purpose will be defeated.
Accordingly, an object of the present invention is to provide a VOA in combination with an electronic feedback loop.
These and other objects of the present invention are provided in an optical communication assembly that includes a demultiplexer coupled to an input fiber, a multiplexer coupled to an output fiber and a plurality of optical fibers. Each optical fiber is coupled to one or both of the demultiplexer and multiplexer. A plurality of attenuators are each coupled to an optical fiber in the plurality of optical fibers.
In another embodiment, an optical communication assembly includes a first optical cross connect coupled to a first portion of a first set of optical fibers and a first portion of a second set of optical fibers. A second optical cross connect is coupled to a second portion of the first set of optical fibers and a second portion of the second set of optical fibers. A first demultiplexer is coupled to a first input fiber and the first portion of the first set of optical fibers. A second demultiplexer is coupled to a second input fiber and the second portion of the first set of optical fibers. A first multiplexer is coupled to a first output fiber and the first portion of the second set of optical fibers. A second multiplexer coupled to a second output fiber and the second portion of the second set of optical fibers. A first set of attenuators is coupled to the first set of optical fibers and a second set of attenuators is coupled to the second set of optical fibers.
In another embodiment, an optical communication assembly includes a demultiplexer coupled to an input fiber, a multiplexer coupled to an output fiber and a plurality of optical fibers. Each optical fiber is coupled to one or both of the demultiplexer and multiplexer. A plurality of attenuators are each coupled to an optical fiber of the plurality of optical fibers. Each attenuator includes an attenuator optical fiber with a longitudinal axis, a core and a cladding in a surrounding relationship to the core. The attenuator optical fiber has multiple cladding modes. The attenuator also has an acoustic wave propagation member with a proximal end and a distal end. The distal end is coupled to the attenuator optical fiber. The acoustic wave propagation member propagates an acoustic wave from the proximal to the distal end and launches an acoustic wave in the attenuator optical fiber. At least one acoustic wave generator is coupled to the proximal end of the acoustic wave propagation member.